Saposin D is a sphingolipid activator protein required for the lysosomal breakdown of ceramide to a fatty acid and sphingosine by acid ceramidase. The crystal structure of saposin D has been determined in two different crystal forms, resulting in a total of six crystallographically independent views of this small 80-amino-acid protein. All of the structures are highly similar and reveal the monomeric form of the saposin fold previously seen in the crystal structures of saposins A and C. Saposin D is slightly more compact than the related saposins A and C owing to a slight repositioning of the 'stem' and 'hairpin' regions of the protein.